1. Field of the Application
The present invention relates to apparatus for abrasive treatment of sheet material, and more particularly to an apparatus for treating a sheet material with the use of ferromagnetic powder.
The invention can be advantageously adapted for application in the treatment of printed-circuit boards for use in various radioelectronic circuits. It also can find utility in other branches of technology where high surface finish quality in sheet materials is required.
2. Description of the Prior Art
An apparatus for treating sheet material with the use of ferromagnetic abrasive powder is known. The apparatus comprises a magnetic system adapted to retain the abrasive powder within the treating zone, and a means for feeding said material to this zone. The magnetic system of this apparatus is formed by two cylindrical rolls made of ferromagnetic material and mounted so as to form a gap therebetween for the passage of the sheet material being treated, a magnetic field exciting means, and magnetic circuit positioned so that the lines of force pass in the direction of axes of the rolls and cross the gap therebetween. The rolls serve as opposite poles of the magnetic system and rotatable in opposite directions. Under the action of magnetic field the grains of ferromagnetic powder adhere to the surfaces of said rolls and rotate together with them. In the course of rotation the powder cuts its way into the surface of the sheet material, thus effecting the finishing action.
The apparatus described above ensures uniform high-quality surface finish of sheet material. The degree of the surface roughness can be controlled by way of adjusting the magnetic field intensity in the treating zone, as well as by appropriate selection of ferromagnetic powder and lubricant. Such apparatus is described in U.S. Pat. No. 4,040,209 British specification No. 1,491,318, French Pat. No. 7,534,673, FRG Application No P-62957, Swiss Application No. 14132, Italian Application No. 2915.sup.o A/15 filed earlier.
However, the apparatus referred to above suffers from the disadvantage that the ferromagnetic powder used in the process is broken down or crushed into finer particles which adhere to the surface of the rotating rolls and continue to participate in the operating process. As this happens, the surface roughness obtained will differ from a specified one, with the roughing value being reduced. This non-uniformity of surface roughness is undesirable in treatment of certain types of articles, such as printed-circuit boards, since reduced surface roughness will cause the photoresist layer, applied to the surface during subsequent operations, to slide off. Therefore, the ferromagnetic powder should be periodically changed in spite of the fact that it still contains a considerable amount of uncrushed grains. This, in turn, raises the production cost of the articles being treated and reduces the operating efficiency of the apparatus.
It is an object of the present invention to provide an apparatus for treatment of sheet material, which will ensure uniform quality and roughness values in the course of treatment of appreciable amounts of sheet material.
Another object of the invention is to provide for more effective use of ferromagnetic powder.
Still another object of the invention is to enhance the operating efficiency of an apparatus for treatment of sheet material.
Finally, it is an object of the invention to provide automatic separation of the crushed ferromagnetic powder and chips taken off the treated material from the powder which is still good for use, as well as prompt recycling of the latter.
These and other objects of the invention are attained in an apparatus for treating sheet material with the use of ferromagnetic powder in magnetic field, comprising a magnetic system formed by two rolls made from a ferromagnetic material and having a gap therebetween for the passage of the sheet material being treated and rotatable in opposite directions, and a means adapted to excite the magnetic field necessary to retain the powder, with the lines of force of said magnetic field passing along the axes of said rolls and across the gap therebetween, wherein, according to the invention, there is arranged underneath the bottom roll a net with the mesh size thereof being slightly less than a minimum permissible size of the powder grains, said net being disposed within the zone of the magnetic field inducing a force required to retain the powder on the surface of said rolls and enabling the grains of said powder to be lifted from the net and onto the surface of the bottom roll.
It is advisable to provide a switch for on-and-off operation of a magnetic field existing means, which should be deenergized after treating a prescribed amount of material, when there is no material in the treating zone, and reenergized when the sheet to be treated is fed into the gap between the rolls, thereby enabling the powder to be transferred from the net onto the roll.
The advantage of the apparatus construction according to the invention lies in that it ensures stable quality of the finished surface throughout the operating period, since the crushed grains of the powder and chips taken off the surface are constantly removed. The stability of a prescribed roughing value is ensured as well, since it is governed by the size of the net mesh which depends upon a prescribed size of the powder grains used. In addition, the provision of the net allows for more effective use of ferromagnetic powder. It should be observed that the separation of powder is effected automatically in the course of operation when there is no material in the treating zone. Taking into account the high rotating speed of the rolls and the relatively low travelling speed of the sheet material, it becomes obvious that the powder removing operation in no way adversely affects the operating efficiency of the apparatus as a whole.
To enable the ferromagnetic powder, poured off when the action of magnetic field is terminated, to get underneath the bottom roll, the net is preferably made bow-shaped and arranged so that the middle of the low portion thereof is found under the middle part of the roll.
Since a certain amount of powder adheres to the feed rollers, the end portions of the net should be arranged therebelow.